Change CursorTreeAdapter to close the cursors rather than deactivating them....

Change CursorTreeAdapter to close the cursors rather than deactivating them. Fix SimpleCursorTreeAdapter to allow a null cursor as an argument.

Not closing the cursors was a bit of a bug since the documenation explicitly says that it would close the cursors. This will get rid of all the warnings that are printed out in the finalizer about the cursor not being closed.